1,720,971 research outputs found
Effects of pipe angular velocity and oven configuration on tube temperature distribution in the radiative heating of PVC pipes
No full textSeveral manufacturing processes in polymer industry aim at obtaining products by deforming preforms or sheets after a heating process. A thorough knowledge of the operating parameters of such heating processes is fundamental to fulfill the often high production requirements with the least energy consumption and to avoid unacceptable defects in the final product. A common example of such an application is the end-forming process of polyvinyl chloride (PVC) tubes, which are enlarged at one end in order to allow pipes connections. The heating phase which comes before the deformation process is usually carried out in ovens equipped with short wave infrared lamps; to ensure uniform heating, pipes rotate with a given angular velocity, which represents a fundamental parameter for the success of the whole manufacturing process. In this work, a transient analysis of the radiative heat exchange between rotating PVC pipes and infrared lamps in an oven for end-forming process has been conducted by means of a finite element model, in order to investigate the influence of cylinder angular velocity on the temperature distribution in the tube. Local view factors have been calculated for different oven configurations and have been expressed as a function of angular velocity, allowing pipe rotation to be simulated as a time-dependent boundary condition, instead of using a moving mesh. Simulations were carried out for different tubes geometries and angular velocities and results were compared with the case of a uniformly irradiated tube in terms of temperature displacement. For a given oven configuration, the results obtained by the numerical model can be used to find a critical angular velocity over which further increase does not lead to appreciable improvements in temperature evenness. The effect of the lampsâ relative position was also investigated, showing a significant influence on critical angular velocities obtained. The model realized represents a potential tool to characterize the end-forming process in terms of critical angular velocity, leading to reductions in machine set-up time and product waste due to thermal failure
Thermal characterization of the end-forming process of PVC pipes: influence of the number of lamps on critical angular velocities
No full textThe end-forming, or belling, of plastic pipes allows them to be joined together to form longer ducts. The first stage of the process entails softening the pipe wall through heating, and defines the properties of the final product. Because of the very low value of thermal conductivity of plastics and to speed heating up, the pipes are placed in ovens whose walls are lined with infra-red short-wave (SW) lamps. The radiation emitted partly penetrates the pipe wall, quickening the process. The heating elements have a straight configuration, and can only be laid axially flush over the oven’s wall, the pipes, therefore, must rotate to obtain a circumferentially uniform heating and avoid damage. The threshold speed to avoid scorching while exceeding a desired temperature over the thickness of the pipe wall is defined as ”critical angular velocity” and is strictly dependent on pipe geometry and oven characteristics such as the number and layout of lamps. The Authors have already investigated the problem extensively, as is well documented in the literature, yet one aspect still remains to be studied, namely the influence of the number of lamps on the heat flux distribution over the pipe’s perimeter and on the critical velocity. In this work, the issue is investigated thoroughly using the same approach previously adopted and recalled in its main aspects in the paper. It is found that even a significant reduction in the number of lamps does not increase threshold velocities to technically unfeasible values. A non-negligible reduction in costs can therefore be achieved without significant impact on the process outcome
Transient analysis of the radiative heating of rotating PVC pipes in a oven for end-forming process
No full textIn polymer industry several applications are aimed at obtaining products by deforming preforms or sheets after a heating process which, if not carried out properly, may lead to unacceptable defects in the final product. A common example of such an application is the end-forming process of polyvinyl chloride (PVC) tubes, in order to allow piping connections. One end of the tube undergoes a deformation process preceded by a heating phase, which is conducted in a oven equipped with infrared lamps. The pipe rotates with a given angular velocity, in order to ensure uniform heating. Hence, the pipe angular velocity represents a fundamental parameter for the proper success of end-forming process. In this work, the transient analysis of the radiative heat exchange between rotating PVC pipes and infrared lamps of a oven for end-forming process has been carried out by means of a finite element model, in order to investigate the influence of cylinder angular velocity on the temperature distribution in the tube. Local view factors were calculated as a function of geometry and oven configuration and were expressed as a function of angular velocity, allowing pipe rotation to be simulated as a time-dependent boundary condition, instead of using a moving mesh. Simulations were carried out for tubes of different geometries and for different values of angular velocity and results were compared with the case of a uniformly irradiated tube. The model realized represents a tool to characterize the end-forming process for different polymeric materials and different oven configurations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Temperature-Dependent Conductances to Improve the Accuracy of the Dynamic Model of an Electric Oven
No full textElectric ovens, with a usual energy efficiency between 10% and 12%, rank among low-efficiency household appliances. The energy classification of such appliances is regulated by the EN 60350-1 European standard [1] which mandates proper control of the oven centre temperature; the design of suitable, advanced strategies thus represents a central field of investigation. In this work, a low-order dynamic model suitable for control design was developed to simulate the transient behaviour of an electric domestic oven in natural convective operation
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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